Mass transfer system

ABSTRACT

A mass transfer machine removes dissolved gases or volatile organic compounds from air or a liquid. The mass transfer machine includes a vessel capable of containing a liquid and having a liquid inlet and an air inlet at a near end. It further includes a liquid outlet at a far end. Two or more baffles are located transversely inside of the vessel. The baffle located nearest to the far end of the vessel has a window. Two or more diffusers are located near a bottom surface of the vessel. The diffusers are in communication with the air inlet and have two or more orifices through a wall. An adjustable plate is releasably mounted over the window of the baffle located closest to the far end of the vessel. The plate is adjustable in a vertical direction. In one embodiment, an air source supplies air to the air inlet located on the near end of the vessel. In another embodiment, an air source is connected to the an air exit located on the far side of the vessel.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. patent application Ser. No.09/543,167, filed Apr. 5, 2000 issued as U.S. Pat. No. 6,355,096 B1 onMar. 12, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to a mass transfer machine for removingmoving dissolved gases or volatile organic compounds from a liquid to agas or from a gas to a liquid. More specifically, the present inventionrelates to a mass transfer system that provides improved flexibility andefficiency.

A mass transfer machine is capable of moving dissolved gases or volatileorganic compounds in either direction. The transfer of mass from aliquid (typically water) to air is commonly referred to as air strippingor degasifying. The transfer of mass from air to a liquid is commonlyreferred to as scrubbing. The use of mass transfer machines to removevolatile compounds from water is known in the art. A variety of masstransfer machines are known, including air strippers, air scrubbers, anddistillation towers. Each of these devices operates under the same basicprinciple. A gas and a liquid are brought into contact with each othercausing dissolved gases or volatile organic compounds to migrate fromthe liquid to the gas.

Mass transfer machines are useful in numerous situations. Typicalapplications include removal of radon or CO₂ from well water andremoving contaminants from water at refueling depots, petro chemicalplants, hazardous waste sites, or landfills. It is thereforeadvantageous that the mass transfer machine be portable and capable ofoperating on-site. Further, it is important that the mass transfermachine operate as efficiently as possible to minimize powerconsumption, while at the same time maximizing removal rates. There is aneed in the art for a portable, low maintenance, energy efficient, masstransfer machine suitable for on-site operation.

BRIEF SUMMARY OF THE INVENTION

The present invention is a mass transfer machine for transferringdissolved gases or volatile organic compounds between air and a liquid.The mass transfer machine includes a vessel having a liquid inlet and anair inlet at a proximal end. The vessel has a liquid outlet at a distalend. The vessel is capable of containing a liquid. The vessel containsat least two chambers, and the chamber located nearest to the distal endof the vessel has an adjustable height passageway used to control theheight of the liquid in the vessel. A diffuser is located near a bottomsurface of each chamber of the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the mass transfer machine of the presentinvention.

FIG. 2 is a top perspective view of the mass transfer machine of thepresent invention.

FIG. 3 is a top perspective view of the mass transfer machine shown inFIG. 2, with the cover removed.

FIG. 4A is a top view of the mass transfer machine shown in FIG. 2, withthe cover removed.

FIG. 4B is a side view of the mass transfer machine shown in FIG. 2.

FIG. 5 is a sectional view of an air diffuser for use with the presentinvention.

FIG. 6 is a schematic view of the mass transfer machine of the presentinvention, showing the liquid flow path.

DETAILED DESCRIPTION

FIG. 1 shows a schematic view of a mass transfer system 10 for removingcontaminants from ground water or waste water. The mass transfer system10 includes, as shown from left to right in FIG. 1, an air source 12, apump 14, a mass transfer machine 16, an exhaust vent 18, and a waterexit 20. The air source 12 is coupled to an air inlet on the masstransfer machine 16 and the exhaust vent 18 is coupled to an air outleton the mass transfer machine 16. The pump 14 is connected to a waterinput, and the water exit 20 is connected to a water outlet on the masstransfer machine 16.

FIG. 2 shows a top perspective view of the mass transfer machine 16. Asshown in FIG. 2, the mass transfer machine 16 includes, on its frontside, an air inlet 22, a water inlet 24, and a drain 26. The air inlet22 and water inlet 24 are each located near the top of the front side ofthe mass transfer machine 16. Each inlet 22, 24 is configured forconnection to a supply pipeline. The drain 26 is located near a bottomof the front side of the mass transfer machine 16 and is configured likethe inlets 22, 24. As further shown in FIG. 2, the mass transfer machine16 includes, on its rear face, an air outlet 28 and a water outlet 30.Both outlets 28, 30 are located near a vertical midline. The air outlet28 is located near the top of the rear face, and the water outlet 30 islocated near the bottom of the rear face. The mass transfer machine 16further includes a body 32 and a cover 34 releasably secured to the body32 by fasteners 36. In one embodiment, the body 32 and the cover 34 ofthe mass transfer machine 16 are constructed from polypropylene by awelding technique.

FIG. 3 shows a top perspective view of the mass transfer machine 16 withthe cover 34 removed. FIG. 3 also shows the air source 12 and theexhaust vent 18 connected to the mass transfer machine 16. As shown inFIG. 3, the mass transfer machine 16 includes a cover 34, which may bereleasably coupled to the body 32 by fasteners 36. As further shown inFIG. 3, located inside the body 32 are a first baffle 38, a secondbaffle 40, and a final baffle 42, which define a first chamber 44, asecond chamber 46, a final chamber 48, and a quiescent chamber 50. Thebaffles 38, 40, 42 extend laterally across the inside of the body 32 andare disposed generally perpendicular to a longitudinal centerline. Asshown in FIG. 3, the first baffle 38 includes an opening 51 located neara left side of the body 32. The opening 51 is positioned on the side ofthe body 32 opposite the side of the water inlet 24. Each successivebaffle, moving from the front to the rear of the body 32, with theexception of the final baffle, includes an opening. The openings areplaced on opposite sides of the body 32. This configuration acts tocreate serpentine flow of the water through the mass transfer machine16, as described in greater detail below. In one embodiment, the baffles38, 40, 42 (like the body 32 and the cover 34) are constructed frompolypropylene. Depending upon the chemicals present in the system, othermaterials could also be used including polyethylene and teflon.

The final baffle 42 includes a weir plate 52, which is adjustable tocontrol the depth of water inside the first chamber 44, the secondchamber 46, and the final chamber 48. The operation of the weir plate 52is discussed in greater detail below. In the embodiment shown in FIG. 3,the mass transfer machine 16 includes three chambers, howeveralternative embodiments of the present invention include a mass transfermachine 16 having four, five, six, seven, eight, nine, or more chambers.The purposes and advantages of having various chamber numbers isdiscussed in greater detail below. On the top surface of the body 32, agasket 54 lies in a groove. The gasket 54 acts in conjunction with thecover 34 and the fasteners 36 to create an air-tight seal between thebody 32 and the cover 34 of the mass transfer machine 16.

FIG. 4A shows a top view of the mass transfer machine 16 with the cover34 removed, and FIG. 4B shows a side view of the mass transfer machine16. As shown in FIGS. 4A and 4B, the air inlet 22 is in communicationwith an air manifold 56. The air manifold 56, in turn, is incommunication with a first diffuser 58 a and a second diffuser 60 alocated in the first chamber 44. The manifold 56 is also incommunication with a first diffuser 58 b and a second diffuser 60 blocated in the second chamber 46. Additionally, the manifold 56 is incommunication with a first diffuser 58 c and a second diffuser 60 clocated in the final chamber 48. In the embodiment of the presentinvention, as illustrated in FIGS. 4A and 4B, each of the chambers 44,46, 48 includes two diffusers 58, 60. In a first alternative embodiment,each of the chambers 44, 46, 48 includes only one diffuser. In a secondalternative embodiment, the chambers 44, 46, 48 include more than twodiffusers per chamber.

As shown in FIGS. 4A and 4B, the diffusers 58, 60 are located near abottom surface of the body 32 and extend substantially along the lengthof the chambers 44, 46, 48. In the illustrated embodiment, the diffusers58, 60 are disposed generally parallel to one another and spacedgenerally equidistant from each other and from the chamber walls. Thediffusers 58, 60 are hollow tubes, each having several diffuser orifices62 extending through the tube wall. In one embodiment, the diffusers 58,60 are constructed from polypropylene. The diffuser orifices 62 aregenerally spaced equal distances from one another and located along astraight line. In a first embodiment, the diffuser orifices 62 arelocated along a line extending along a top surface of the diffusers 58,60. In alternative embodiments, the diffuser orifices 62 may be locatedalong a line other than that defined by the top surface of the diffusers58, 60, or the orifices may be located along multiple lines runningalong the surface of the diffusers 58, 60.

The diffusers 58, 60, are shown in greater detail in FIG. 5. Thediffuser 58 includes a proximal end 64 and a distal end 66. The diffuser58, shown in FIG. 5, includes a diffuser sleeve 68 surrounding theexternal surface of the diffuser 58 and covering the diffuser orifices62. The diffuser sleeve 68 extends from the distal end 66 to near theproximal end 64 of the diffuser 58. A clamp 70 is used to secure thediffuser sleeve 68 to the diffuser 58 and form an air-tight seal. In oneembodiment, the clamp 70 is a stainless steel band clamp. The diffuser68 includes sleeve orifices 72 extending through a wall of the diffusersleeve 68. In a first embodiment, the diffuser sleeve 68 is made from apolymeric material. In a second embodiment, the diffuser sleeve 68 ismade from EPDM. The sleeve orifices 72 on the diffuser sleeve 68 are ofa smaller diameter than the diffuser orifices 62 on the diffuser 58.

During operation of the mass transfer system 10, the water from whichthe dissolved gases or volatile organic compounds are to be removed issupplied to the water inlet 24 located on the front surface of the body32 of the mass transfer machine 16. The contaminated water is suppliedto the water inlet 24 by the pump 14. The pump 14 is sized appropriatelyfor the rate at which the water is to be supplied to the water inlet 24.The contaminated water then enters the body 32 of the mass transfermachine 16. FIG. 6 shows the flow path of the contaminated water throughthe mass transfer machine 16. As shown in FIG. 6, the water enters thefirst chamber 44, flows into the second chamber 46, flows into the finalchamber 48, flows over the weir plate 52 into the quiescent chamber 50,and finally flows out through the water outlet 30. At this point, thedrain 26 is closed to prevent water from exiting at the proximal end ofthe mass transfer machine 16.

As illustrated in FIG. 6, the contaminated water travels through themass transfer machine 16 in a serpentine flow path. This serpentine flowpath is generated by the placement of the openings in the baffles of themass transfer machine 16. The first baffle 38 has an opening located atthe side of the mass transfer machine 16 opposite that of the waterinlet 24. The second baffle 40 has an opening at the side of the masstransfer machine 16 opposite to that of the first baffle 38. Anysuccessive baffles will have openings at alternating opposite sides ofthe mass transfer machine 16. The final baffle 42 has an opening at theside opposite the opening in the previous baffle. The height of theopening in the final baffle 42 may be adjusted using the weir plate 52,thereby controlling the contaminated water depth throughout the masstransfer machine 16.

The weir plate 52 may be secured to the final baffle 42 in a variety ofmanners. In one embodiment, the weir plate has a series of holes runningalong each vertical edge, and the final baffle 42 has one hole locatedon each side of the window. The weir plate 52 is then fastened to thefinal baffle 42 by inserting a bolts through a hole in each side of theweir plate 52 and through the hole in each side of the final baffle 42.In an alternative embodiment, the weir plate 52 has a groove runningalong each vertical edge. Adjustment of the height of the weir plate isthen made by loosening a fastener, sliding the weir plate 52 to thedesired height, and tightening the fastener. In another alternativeembodiment the water level is controlled by an adjustable heightpassage, which performs the same function as that of the weir plate 52.

While the contaminated water is flowing in a serpentine manner throughthe mass transfer machine 16, the air source 12 supplies air to the airinlet 22 located on the body 32 of the mass transfer machine 16 (asshown in FIG. 3). The air source 12 is typically either a fan or ablower, as will be described in greater detail below. Air provided bythe air source 12 enters the mass transfer machine 16 through the airinlet 22, travels into the manifold 56 (shown in FIGS. 4A and 4B),enters the array of diffusers 58, 60, and finally exits the diffusersthrough the diffuser orifices 62. The diffuser orifices 62 are sized,using techniques known in the art, to create air bubble sizesappropriate for removal of dissolved gases or volatile organic compoundsfrom the contaminated water. The diffuser orifices 62 are also generallysized large enough to prevent fouling by particles in the air. The airbubbles then enter the contaminated water and flow up and out the topsurface of the water. In a first embodiment, the air is then free toexit the mass transfer machine 16 through a top surface. In a secondembodiment, the cover 34 of the mass transfer machine 16 is secured tothe top of the body 32 by fasteners 36, in such a manner as to form anair-tight seal. In this embodiment, the only exit path for the air isthrough the air outlet 28 and out the exhaust vent 18. The benefit ofthis embodiment is that the off-gas may be then treated on site. Whetheror not the off-gas exiting through the exhaust vent 18 is treatedtypically depends on what type of contaminant the air has removed fromthe water. After operation, any water remaining in the mass transfermachine 16 may be released by opening the drain 26 located near thebottom of the mass transfer machine 16.

Because of the flow path and the storm of bubbles from the diffusers,the flow of liquid through the chambers 44, 46, 48 of the mass transfermachine 16 occurs with complete mixing in each chamber and plug flowfrom one chamber to the next. This flow pattern results in a sequentialbatch-type flow model, which helps maximize mixing of the contaminant inthe liquid and maintain a high concentration gradient. Further, asopposed to tower systems known in the prior art, in the presentinvention, the air is delivered to the mass transfer machine 16 inparallel. In other words, each chamber 44, 46, 48 of the mass transfermachine 16 is exposed to clean, uncontaminated air, as opposed torecycling the same air from one chamber to the next. This parallel airflow helps to maximize the concentration gradient between the air andthe liquid.

The mass flow rate at which dissolved gases or volatile organiccompounds is transferred from the contaminated water to the air is afunction of several variables, including the concentration level ofcontaminant in the water, the particular contaminant that is beingremoved, the size of the air bubbles exiting from the diffusers 58, 60the number of air bubbles passing through the liquid, and the exposuretime. The objective of the mass transfer machine 16 is to create thehighest possible air to water interface, as determined by the number andsurface area of the air bubbles, with the lowest possible powerconsumption. An advantage of the present invention, therefore, lies inits flexibility, which allows it to operate at the lowest possible powerconsumption for a given removal percentage.

In a first preferred embodiment, the air is provided to the masstransfer machine 16 by a fan. The fan is connected to the air inlet 22of the mass transfer machine 16 through a throttle. This throttle,typically a blast-gate throttle, allows the air flow rate delivered tothe air inlet 22 to be adjusted. In this first preferred embodiment,water is supplied to the mass transfer machine 16 by a pump 14 andtravels through the mass transfer machine 16 in the serpentine fashiondescribed above. The weir plate 52 may be adjusted up or down to controlthe depth of the contaminated water in the mass transfer machine 16.Increasing the depth of the contaminated water increases the residencetime and thus increases the amount of contaminant removed (i.e., theamount of mass transferred). At the same time, however, increasing thedepth of contaminated water increases the pressure required to move airthrough the system and, therefore requires more energy. Duringoperation, as the weir plate 52 is raised to increase the depth ofcontaminated water in the mass transfer system 16, the throttle, locatedbetween the fan and the air inlet 22, is opened to decrease the pressuredrop and increase the amount of air delivered by the fan. The presentinvention thereby allows an operator to minimize the power consumptionof the mass transfer system 10 necessary to achieve a specifiedcontaminant removal percentage.

In some applications, off-gas treatment is a critical part of thecleansing process. In those situations where it is necessary ordesirable to treat the off-gas, it is advantageous to be able tominimize the volume of air flow used to transfer contaminant from thewater to the air, because less air will then need to be treated. It ispossible to achieve the same mass transfer rates with lower air flows bydecreasing the size of the bubbles exiting from the diffuser. Morebubbles, each bubble having a smaller diameter, increases the amount ofsurface area for a given air volume. In the present invention, smallerdiameter air bubbles are generated by using diffusers having smallerorifices.

As explained above, in the present invention, a diffuser sleeve may beplaced over the diffusers 58, 60. The diffuser sleeves 68 are made froman EPDM material having significant flexibility. This flexibility allowsthe sleeve and the orifices to expand and contract which has anadvantage of preventing fouling of the orifices. Use of the diffusersleeve 68, however, results in the need for an air supply under greaterpressure. In this embodiment of the present invention, therefore, theair source 12 is an appropriately sized blower capable of generating therequired pressure. In this embodiment, the power consumption isincreased to allow the realization of the benefit of a smaller volume ofoff-gas requiring treatment. During operation of the present invention,when used in conjunction with a diffuser sleeve 68, air exits thediffusers 58, 60 through the diffuser orifices 62 and enters a manifoldbetween the diffusers 58, 60 and the diffuser sleeves 68. When airpressure in this manifold reaches a sufficiently high level, the airbubbles through the sleeve orifices 72 and enters the contaminatedwater.

In an alternative embodiment of the present invention, the cleansing airis moved through the mass transfer machine 16 using an induced draftmethod. In the induced draft method, the air source 12 (typically eithera fan or a blower) is connected to the air outlet 28. Operation of thefan or blower, in this configuration, generated a negative air pressureinside mass transfer machine 16 (assuming that the cover 34 has beenattached to the body 32 to form an air-tight seal), which causes air tobe drawn in through the air inlet 22, into the manifold 56, into thediffusers 58, 60, and through the orifices 62. This induced-draft methodprovides the advantage of preventing the situation where pressureaccumulated inside the mass transfer machine 16, possibly leading to anexplosion involving the volatile organic compound.

It is important to note that, while the above discussion relates to thetransfer of diffuses gases or volatile organic compounds from a liquidto air, the present invention will operate equally as well fortransferring mass from air to a liquid. Transfer of mass from air to aliquid is commonly done both to clean or scrub the air and to add adissolved gas to the liquid (typically water). The ease at which thetransfer of mass will occur in one direction or the other is a functionof the solubility of the gas or volatile organic compound in water.Highly soluble substances will move more easily from air into water,while low solubility substances will move more easily from water intoair. The present invention operates to move mass from air to liquid inthe same manner as described above. The gas or volatile organic compoundwill move from the air to the water if the concentration gradientcompels movement in that direction. In this mode of operation,contaminated air will be supplied to the diffusers, and clean water willbe supplied to the mass transfer machine 16.

Another feature of the present invention that provides improvedflexibility is the ability to add additional chambers. Adding additionalchambers increases the residence time of the contaminated water in themass transfer system 16 and increases the exposure to air bubbles fromthe diffusers 58, 60. These two factors increase the amount ofcontaminant removed from the water. Additionally, if a higher removalpercentage is needed than one mass transfer system 10 is capable ofproviding, two or more mass transfer systems 10 may be connected in aseries. A series connection of multiple mass transfer machines 16 willresult increased removal percentages. Alternatively, if highercontaminated water throughput rates are needed, two or more masstransfer machines 16 may be coupled to the contaminated water source inparallel. Connection of mass transfer machines 16 in parallel allows fora higher contaminated water throughput at the same removal percentage.

A further advantage of the present invention is its low maintenancedesign. The presence of a cover 34 that is releasably securable to thebody 32 allows for easy access and cleaning of the inside of the masstransfer machine 16. Further, the design of the mass transfer machine 16includes no moveable parts during operation. In one embodiment of thepresent invention, the mass transfer machine 16 is constructed entirelyfrom polypropylene and stainless steel. This design provides theadvantage of high corrosion resistance, while maintaining stability uponexposure to dissolved gases or volatile organic compounds.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A method of transferring dissolved gases orvolatile organic compounds between air and a liquid, the methodcomprising: supplying liquid to a vessel wherein the liquid flowshorizontally through the vessel and the vessel is comprised of aplurality of transversely disposed interconnected chambers; injectingair into the liquid through orifices in a plurality of diffusers;controlling a height of the liquid within the chambers of the vessel,wherein controlling the height of the liquid is performed by anadjustable height passage located at an exit passageway of one of thechambers; and controlling a flow rate and a pressure of the air injectedinto the liquid.
 2. The method of claim 1 wherein supplying liquid tothe vessel is performed by a pump.
 3. The method of claim 1 wherein theliquid has a serpentine flow through the interconnected chambers of thevessel.
 4. The method of claim 1 wherein baffles define theinterconnected chambers, each baffle having an opening at one end toallow the liquid to flow from chamber to chamber.
 5. The method of claim1 wherein injecting air into the liquid is performed by a blower incommunication with an air inlet of the vessel.
 6. The method of claim 1,and further comprising placing a diffuser sleeve over at least one ofthe plurality of diffusers to create a manifold external to the orificesof at least one diffuser, the diffuser sleeve having a plurality ofsleeve orifices of a diameter different than a diameter of the orificesof the at least one diffuser.
 7. The method of claim 1, and furthercomprising capturing the air exiting the vessel through an exhaust vent.8. The method of claim 7, and further comprising treating the airexiting the exhaust vent to remove hazardous components.
 9. The methodof claim 1 wherein injecting air into the liquid is performed by a fanin communication with an air outlet of the vessel to create a negativepressure in the vessel causing air to be drawn through an air inlet intothe vessel.
 10. The method of claim 1 wherein the adjustable heightpassage is controlled by a weir plate.
 11. The method of claim 1 whereincontrolling the flow rate and the pressure of the air is performed by athrottle.
 12. A method of transferring dissolved gases or volatileorganic compounds between air and a liquid, the method comprising:supplying liquid to a vessel wherein the liquid has a serpentine andsubstantially horizontal flow through the vessel; injecting air into theliquid through orifices in a plurality of diffusers; controlling aheight of an adjustable exit passageway from the vessel to vary a heightof the liquid in the vessel; and controlling a flow rate and a pressureof the air injected into the liquid.
 13. The method of claim 12 furthercomprising placing a diffuser sleeve over at least one of the diffusersto create a manifold external to the orifices of the at least onediffuser, the diffuser sleeve having a plurality of sleeve orifices of adiameter different than the diameter of the orifices of the at least onediffuser.
 14. The method of claim 13 wherein injecting air into thewater is performed by a blower.
 15. The method of claim 12, and furthercomprising capturing air exiting the vessel through an exhaust vent andtreating the air to remove hazardous components.
 16. The method of claim12 wherein injecting air into the water is performed by using a fanlocated at an exit to the vessel to create a negative pressure in thevessel causing air to be drawn through an inlet to the vessel from theoutside.
 17. The method of claim 12 wherein controlling the flow rateand the pressure of the air is performed by a throttle.
 18. The methodof claim 12 wherein the vessel is comprised of a plurality oftransversely disposed interconnected chambers.
 19. The method of claim18 wherein baffles define the interconnected chambers, each bafflehaving an opening at one end to allow the liquid to flow from chamber tochamber.
 20. The method of claim 12 wherein controlling the height ofthe adjustable exit passageway is performed by a weir plate located inthe adjustable exit passageway.
 21. A method of transferring dissolvedgases or volatile organic compounds between air and a liquid, the methodcomprising: supplying liquid to a vessel wherein the liquid flowshorizontally through the vessel and the vessel is comprised of aplurality of transversely disposed interconnected chambers; injectingair into the liquid through orifices in a plurality of diffusers;controlling a height of an adjustable height plate located in an openingof one of the chambers to vary a height of the liquid within the vessel;and controlling a flow rate and a pressure of the air injected into theliquid.
 22. The method of claim 21 wherein baffles define theinterconnected chambers, each baffle having an opening at one end toallow the liquid to flow from chamber to chamber.
 23. The method ofclaim 22 wherein the liquid has a serpentine flow through theinterconnected chambers of the vessel.
 24. The method of claim 21wherein the adjustable height plate is a weir plate.
 25. The method ofclaim 21, and further comprising placing a diffuser sleeve over at leastone of the plurality of diffusers to create a manifold external to theorifices of at least one diffuser, the diffuser sleeve having aplurality of sleeve orifices of a diameter different than a diameter ofthe orifices of the at least one diffuser.
 26. The method of claim 21,and further comprising capturing air exiting the vessel through anexhaust vent and treating the air to remove hazardous components. 27.The method of claim 21 wherein injecting air into the liquid isperformed by a blower in communication with an air inlet of the vessel.28. The method of claim 21 wherein injecting air into the liquid isperformed by a fan in communication with an air outlet of the vessel tocreate a negative pressure in the vessel causing air to be drawn throughan air inlet into the vessel.
 29. The method of claim 21 wherein theopening of at least one of the chambers is an exit passageway from thevessel.